Outer blade cutting wheels for cutting rare earth permanent magnets are disclosed in JP-A 9-174441, JP-A 10-175171, and JP-A 10-175172 as comprising a cemented carbide base having an outer periphery to which diamond abrasive grains are bonded with phenolic resins or the like. Since diamond grains are bonded to the cemented carbide base, the base has been improved mechanical strength over prior art alloy tool steel and high-speed steel bases, leading to a higher accuracy of machining. Also by reducing the thickness of the blade with using a cemented carbide base, the yield of machining can be improved and the machining speed be accelerated. While these cutting wheels using cemented carbide bases show better cutting performance than prior art outer blade cutting wheels, the market poses an increasing demand to reduce the cost of cutting wheels. It would be desirable to have a novel high-performance cutting-off wheel overwhelming the prior art outer blade cutting wheels.
While various cutting techniques including outer blade, inner blade and wire saw cutting-off techniques are implemented in machining rare earth permanent magnets or sintered magnets, the outer blade cutting-off technique is most widely employed. By virtue of many advantages including an inexpensive cutting wheel machine, an acceptable cutting allowance on use of cemented carbide blade, a high accuracy, a relatively high machining speed, and a mass scale of manufacture, the outer blade cutting-off technique is widely employed in cutting of rare earth sintered magnets.
Traditional cutting wheels for outer cutting used bases made of steel alloy materials such as alloy tool steels (e.g., SKD grade in JIS) and high-speed steels. However, JP-A 9-174441, JP-A 10-175171, and JP-A 10-175172 (the inventors including the same as the present) disclose cutting wheels using bases of cemented carbides. Cemented carbides made by cementing tungsten carbide (WC) grains in a binder matrix of cobalt or nickel metal by sintering are high-rigidity materials having a Young's modulus as high as 450 to 700 GPa and extraordinarily stronger than the steel alloy materials having a Young's modulus of the order of 200 GPa.
A high Young's modulus implies that the quantity of deformation of a blade under a cutting force (or cutting resistance) is reduced. This, in turn, implies that under the same cutting force, the deflection of the blade is reduced, and that for the same deflection of the blade, the same accuracy of cutting is possible even when the thickness of the blade is decreased. Although the cutting force applied per unit area of the blade remains substantially unchanged, the overall cutting force applied to the blade becomes smaller by the thickness decrease. In the multiple machining process where a magnet block is machined into multiple pieces at a time by a cutter assembly comprising a multiplicity of cutting wheels, the total cutting force applied to the cutter assembly is reduced. This allows the number of cutting wheels to be increased for a motor of the same power, or the cutting force to be reduced for the same number of cutting wheels, leading to a saving of the motor power. If the motor power has a margin relative to the cutting force, the advance of the cutting wheel assembly may be accelerated to shorten the cutting time.
The use of high-rigidity cemented carbide bases considerably improved the productivity of outer blade cutting. However, the market imposes an ever increasing demand on rare earth sintered magnets, with manufacturers entering into keen competition toward cost reduction. For effective utilization of rare earth sintered magnet material, the smaller the cutting allowance, the higher becomes the material utilization yield. The higher the machining speed, the more is improved the productivity. It would be desirable to have an outer blade cutting wheel which offers a high rigidity and high accuracy despite a reduced thickness of blade relative to the current cemented carbide base cutting wheels.